Middling black hole may be hiding in star cluster

Medium-weight black holes may exist after all. Astronomers say signs of a black hole with about 2,200 times the mass of the sun have been detected at the center of the star cluster 47 Tucanae. If confirmed, the discovery could hint at a new class of black holes, ones starved of gas.

“It’s notoriously challenging to observe the centers of star clusters,” says Bülent Kiziltan of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. “We looked at how the stars in the cluster were being stirred up. Their dynamics suggest there has to be a black hole there.” A study appearing online February 8 in Nature describes the intermediate-mass black hole.

Kiziltan and colleagues studied pulsars, neutron stars that rotate rapidly and emit a beam of electromagnetic radiation. Observers on Earth see the beam as a regularly pulsed signal that can be used to track the movements of the stars over time. Globular clusters such as 47 Tucanae are so dense that heavier stars sink toward the center and should continue to sink inward over time. But that’s not what was happening with the pulsars. They were farther from the cluster center than expected, in a configuration that can only be shaped by a black hole hiding in 47 Tucanae’s core, the team concluded.

The team’s method is “pretty innovative,” says Stephen Zepf, an astronomer at Michigan State University in East Lansing. It could be a new way to detect mid-mass black holes in other globular clusters, if they have pulsars, he says.

Intermediate-mass black holes are 100 to 100,000 times the mass of the sun. If they exist, they would fill the evolutionary gaps between black holes a few times the mass of the sun and those that are millions to billions of times more massive than the sun, hinting at how the supermassive ones grew to be so big. But finding mid-mass black holes has been hard. Superbright X-rays seen in other galaxies hint at a couple of candidates.

But not all black holes may emit X-rays or radio waves, Kiziltan says. The one in 47 Tucanae could have been missed for decades because it created a cocoon around itself, so that very little gas interacts with it and little, if any, light is emitted. This would be very different from how astronomers expect black holes to work. They expect gas to be falling onto black holes all the time, and when it does, there should be some sign, such as a radio signal. But there’s been no such signal from 47 Tucanae. So, Kiziltan and colleagues argue, the black hole there may be a member of a class gas-starved, electromagnetically invisible black holes, which could serve as the seeds of the supermassive black holes that ultimately end up at the center of galaxies.

“Many more of these gas-starved black holes may be out there,” Kiziltan says. He is now using the pulsar technique to look for them.